Surface Heat Transfer Coefficients Equations for Windows

Heat transfer through a window is also affected by the convection and radiation
heat transfer coefficients between the glass surfaces and surroundings.
The effects of convection and radiation on the inner and outer
surfaces of glazings are usually combined into the combined convection
and radiation heat transfer coefficients hi and ho, respectively. Under still
air conditions, the combined heat transfer coefficient at the inner surface of
a vertical window can be determined from:

where Tg = glass temperature in K, Ti = indoor air temperature in K, εg =
emissivity of the inner surface of the glass exposed to the room (taken to be
0.84 for uncoated glass), and σ = 5.67 x 108 W/m2 · K4 is the Stefa
Boltzmann constant. Here the temperature of the interior surfaces facing
the window is assumed to be equal to the indoor air temperature. This assumption
is reasonable when the window faces mostly interior walls, but it
becomes questionable when the window is exposed to heated or cooled
surfaces or to other windows. The commonly used value of hi for peak load
calculation is:

hi = 8.29 W/m2 · °C = 1.46 Btu/h · ft2 · °F (winter and summer)

which corresponds to the winter design conditions of Ti = 22°C and
Tg = -7°C for uncoated glass with εg = 0.84. But the same value of hi can
also be used for summer design conditions as it corresponds to summer conditions
of Ti = 24°C and Tg = 32°C. The values of hi for various temperatures
and glass emissivities are given in the Table below. The commonly used
values of ho for peak load calculations are the same as those used for outer
wall surfaces (34.0 W/m2 · °C for winter and 22.7 W/m2 · °C for summer).

Combined convection and
radiation heat transfer coefficient
hi at the inner surface of a vertical
glass under still air conditions
(in W/m2 · °C)*